Since abt a year ago my hair has been falling out a lot, noticed every time I dyed my hair and even more after I had covid, then loads since I got a keratin treatment . I had blood tests done but everything was fine. Tried different shampoos, biotin and vitamin C tablets. I don't think it's because it's too damaged because the hairdressers told me it's healthy. The hair falls out from the root. Any advice?